Heat-convertible products obtained by reacting amino- or imino-group-containing compounds, e.g., ureas, amides, aminotriazines, and the like, with aldehydes, e.g., formaldehyde, benzaldehyde, etc. have been known for a number of years. Resins obtained by curing such condensation products, e.g., under the influence of heat, possess an excellent combination of physical properties and are widely used in glues, in molding compounds, as finishes for paper and textiles and as surface coatings. The convertible resins can be used per se or they can be further modified before curing, e.g., by alkylation with an alcohol, e.g., methanol or butanol, to provide for solubility and compatibility and/or by admixture with other materials capable of co-reacting therewith, such as compounds containing hydroxyl groups and carboxyl groups e.g., glycols, alkyd resins, polyester resins, and the like. This invention broadly is concerned with amino resins which are suitable for all conventional purposes. However, in its most preferred aspects, it is concerned with soluble forms or liquid forms of such amino resin products, which are well known to be superior as coatings for metals, and coatings or impregnants for cloth, paper, and the like. Such convertible resins commonly comprise urea-or melamine-aldehyde condensates or reaction products thereof with alcohols, e.g., methylol ureas, methylol melamines, and alkylated, e.g., methylated and butylated, derivatives thereof either alone or in a suitable solvent therefor. These specific amino resins are applied by coating onto three-dimensional substrates, e.g., metal, glass, plastics, such as appliance bodies, plastic windows, and the like, and then curing under the influence of heat. The mechanism of cure contemplated is by condensation and cross-linking to split out H.sub.2 O or ROH or HCHO, etc., and curing can be effected without a catalyst if long enough heating times -- of the order of hours and days -- are provided. However, for immediate curing, or for curing at more moderate temperatures, an acid is often added to function as a cross-linking catalyst. Among the acidic catalysts that have been used in the past with amino resins can be mentioned boric acid, phosphoric acids, acid sulfates, hydrochlorides, ammonium phosphates and polyphosphates, acid salts of hexamethylene tetramine, phthalic acid, oxalic acid, and the like. It is also known that sulfonic and sulfonyl halides are especially effective as catalysts in such compositions. A substantial number of such sulfonic acid compounds are illustrated in the Dannenberg patent, U.S. Pat. No. 2,631,138. All of them are of low to moderate molecular weights -- none of them approach a molecular weight of 500. Moreover, while monoalkylaromatic polysulfonic acids and alkylaromatic polysulfonic acids are disclosed, there is no disclosure of polyalkylaromatic polysulfonic acids. The preferred catalyst as shown in the working examples of Dannenberg is p-toluenesulfonic acid, a monoalkylaromatic monosulfonic acid of moderate molecular weight, i.e., 172. U.K. Pat. No. 769,958 also discloses amino resin compositions containing curing catalysts, and specifically mentions as a preferred catalyst, p-toluenesulfonic acid. Coney et al, U.S. Pat. No. 3,265,645, also discloses compositions containing amino resins and states a specific preference for p-toluenesulfonic acid as the curing agent.
It has now been unexpectedly discovered that if compositions containing an amino resin are cured with high molecular weight polyalkylaromatic polysulfonic acid catalysts, specifically those having a molecular weight of greater than about 500, e.g., dinonylnaphthalene disulfonic acid, and the like, curing occurs much more rapidly at conventional temperatures than is achieved in the prior art at corresponding levels of catalyst. Moreover, the catalyzed compositions cure to thermoset systems which have superior properties in comparison with those cured with the catalysts of the prior art, possibly because the new catalysts become chemically bound into the resin, and for other reasons, e.g., improved compatibility.
While it is not clearly understood at this time why the above observed advantageous results have been obtained, it seems possible that by virtue of their unique solubility in both aqueous and non-aqueous systems, their high functionality, their high molecular weight, and inherent isomer distribution (because of the way in which they are made), polyalkylaromatic polysulfonic acids of such high molecular weight are superior to all conventional catalysts in a variety of amino resin systems over a wide range of concentrations. For example, it is known in coating technology that low molecular weight additives, including catalysts, can cause imperfections known as "fish-eyes" or "craters" in the cured system. This has been found to be alleviated by the use of high molecular weight polyalkyl-aromatic polysulfonic acids. Moreover, conventional catalysts such as p-toluene sulfonic acid have a tendency to crystallize out of certain systems during cure, and this mars the appearance of the cured resin. On the other hand, high molecular weight polyalkylaromatic polysulfonic acids are capable of having a very broad isomer distribution and unique solubility characteristics, -- being soluble both in polar and non-polar solvents. They crystallize very slowly -- if at all -- and are superior in preventing marred appearances. Polysulfonic acids also, themselves, are capable of reacting with amino resins to form sulfomethylated covalently bonded structures. A monosulfonic acid can only form chain terminations, and therefore, it is not always desirable to use high concentrations of monosulfonic acids, such as the preferred p-toluenesulfonic acid of the prior art, with amino resin systems. However, polysulfonic acids are capable of giving chain extension in sulfomethylation, and this appears to lead to cured resin networks with superior physical properties, even if high sulfonic acid levels are used. At the same time, the high molecular weight and sluggish crystallization of the polyalkylaromatic polysulfonic acids appear to provide flexibility and toughness in all amino resin systems even if higher than usual concentrations are employed. Experiments have shown that in the instant polysulfonic acids, one of the acid groups is much stronger than the other(s). This is believed to have an important bearing on the unexpectedly good results obtained herein.